1. Field of the Invention
The present invention relates to the field of semiconductor processing and more specifically to a cleaning solution and a method of using the cleaning solution in a single wafer cleaning process.
2. Discussion of Related Art
Wet etching and wet cleaning of silicon wafers is usually done by immersing silicon wafers into a liquid. This is also sometimes done by spraying a liquid onto a batch of wafers. Wafer cleaning and etching is traditionally done in a batch mode where several wafers (e.g. 50-100 wafers) are processed simultaneously. A typical cleaning sequence consists of HF-SC1-SC2. HF (HydroFluoric acid) is a dilute HF solution used for etching thin layers of oxide. This is typically followed by the Standard Clean 1 (SC1 solution) that consists of a mixture of NH4OH, H2O2, and H2O. Sometimes the SC1 solution is also called the APM solution, which stands for Ammonia hydrogen Peroxide Mixture. The SC1 solution is mainly used for removing particles and residual organic contamination. The SC1 solution, however, leaves metallic contaminants behind.
The final solution is the Standard Clean 2 solution (SC2) that is a mixture of HCl, H2O2, and H2O. Sometimes the SC2 solution is also called the HPM solution, which stands for Hydrochloric acid hydrogen Peroxide Mixture. The SC2 solution is mainly used for removing metallic contamination. The particular sequence of SC1 and SC2 is most often referred to as the RCA (Radio Corporation of America) cleaning sequence. Between the HF, SC1, and SC2 solutions there is usually a DI (de-ionized) water rinse. There is usually a DI water rinse after the SC2 solution.
The total time for a standard clean cycle is on the order of 64-70 minutes as shown in FIG. 1a. The HF step takes approximately 1-5 minutes. The SC1 step typically takes 10 minutes and the SC2 step also typically takes 10 minutes. The intermediate and final DI water rinse steps take about 8-10 minutes. The final drying of the wafers typically takes between 10-15 minutes. Typically 50-100 wafers are processed at the same time. If separate baths are used for different chemicals then after one batch with 50-100 wafers leaves a bath, a new batch 50-100 wafers can be loaded. Usually the rate limiting step is the dryer which takes up to 15 min. This means that roughly every 15 minutes a new batch of either 50-100 can be processed resulting in an overall throughput for the system of between 200-400 wafers per hour, respectively for batches of 50 or 100 wafers.
Because there is a need for shorter cycle times in chip manufacturing, there is a need for a fast single wafer cleaning process. In order to make a single wafer cleaning process economical, the processing time per wafer should be on the order of two minutes. This means the entire HF-SC1-SC2 sequence, which normally requires about 64-70 minutes, must be completed within two minutes and at least within three minutes. Unfortunately, presently it is not possible to perform an SC1-SC2 cleaning sequence in less than two minutes and at least within three minutes. Until now, wet processing is usually done in a batch mode, since the throughput of single wafer processing cannot compete with batch processing.
Thus, what is desired is a method of reducing the SC1 and SC2 cleans from the normal processing time to less than or equal to 1½ minutes. It is also required to reduce the time necessary for the HF step and the dry. The present invention shows how to reduce the time of the SC1-SC2 sequence from roughly 40 min down to 1½ min for use in a single wafer fashion and at least less than three minutes for the entire cleaning cycle including HF, cleaning, rinsing and drying.
The cleaning of reclaimed wafers has also been done in the past using batch cleaning. In a typical process of reclaiming a test wafer, the wafer is first chemically polished to remove any films from the surface of the semiconductor wafer. The wafer is then polished using chemical mechanical polishing to further remove any films from the surface with a slurry and a mechanical polishing pad. The wafer is then cleaned by a batch cleaning method where multiple wafers are immersed in cleaning solutions in baths where the wafers are lined up vertically in rows. These methods typically result in various grades of reclaimed wafers that have different levels of contamination in the form of particles and residues. The source of much of the contamination is due to cross-contamination from the batch cleaning processes. The yield of high-grade reclaimed wafers is therefore low.